1. Field of the Invention
The present invention relates to communication systems, and more specifically, to demodulation of multi-carrier modulated signals, such as orthogonal frequency division multiplexed (OFDM) signals.
2. Description of the Related Art
In communication systems, a transmitted signal might not be received by a receiver for various reasons, such as a lack of a line-of-sight between the transmitter and receiver. Additionally, in some instances, the transmitted signal might be received; however, the signal quality might be reduced due to the effects of fading, noise, or other interference. In order to maintain continuity in the transmission and reduce the effects of interference, a communications system may transmit multiple versions of a data stream using transmit diversity techniques, such as spatial diversity, temporal diversity, and frequency diversity. The following background discusses a satellite digital audio radio service (SDARS) system, such as that operated by Sirius Satellite Radio Inc., that employs all three of these transmit diversity techniques.
FIG. 1 shows a simplified diagram of one implementation of a SDARS system 100. Program information, such as music and talk radio, is generated at a studio 102 and provided to a remote uplink site 104, which uplinks the information to a first Sirius Satellite Radio satellite 108 and a second Sirius Satellite Radio satellite 110. The first satellite 108 broadcasts a first time-division multiplexed (TDM) version of the program information over a first frequency range. The second satellite 110 broadcasts a second TDM version of the program information over a second frequency range (frequency diversity), where the second TDM version is delayed with respect to the first TDM version (temporal diversity). In addition, the program information is directly uplinked from the studio 102 to a very small aperture terminal (VSAT) satellite 106. The VSAT satellite 106 provides the program information to terrestrial repeaters 112, which broadcast the information over a third frequency range in a coded orthogonal frequency division multiplexed (COFDM) format. Service subscribers receive zero, one, or more of the broadcast signals using stationary or mobile receivers (e.g., 114). Each signal received traverses a separate path from the respective satellite or terrestrial repeater to the receiver (spatial diversity), and the particular signals received depend on the position of the receiver, the positions of the satellites, and the positions of the terrestrial repeaters.